Moldings comprising a polymeric material which has been provided with a sheet are known to the skilled worker. Instead of laminating or adhering a sheet to plastic components, an increasing trend nowadays, in industrial applications as well, is to switch to backing the sheets with the polymeric material directly in the shaping mold, by injection or compression molding or foaming (A. Grefenstein, “Folienhinterspritzen statt Lackieren, Neue Technik für Karosseriebauteile aus Kunststoff” [Injection backmolding of sheets instead of coating: New technology for plastic bodywork components] in Metalloberflache, 10/99, vol. 53, Carl Hanser Verlag, Munich, 1999).
The multilayer color and/or effect sheets used for producing the moldings comprise, as is known, a backing sheet, at least one color and/or effect basecoat and a clearcoat. They correspond in their construction to the conventional multilayer color and/or effect paint systems.
Particularly in the field of automobile painting, however, a multiplicity of requirements are imposed on the appearance of the molding's surfaces on the sheet side (cf., e.g., European patent EP 0 352 298 B1, page 15 line 42 to page 17 line 40).
The solutions proposed in the prior art, however, do not allow adequate meeting of these requirements, requirements which are usual in the field of automobile painting. Moreover, some of the solutions proposed in the prior art are even contradictory in respect of the glass transition temperature that is to be set for the radiation-crosslinkable clearcoat materials used.
For instance, WO 00/63015 discloses a process of the aforementioned kind for producing moldings, in which the crosslinkable coating composition (K) is composed of a radiation-crosslinkable material which comprises a binder having a glass transition temperature of above 40° C. The transparent coatings (KE), crosslinked to completion, that are obtained in this process have unsatisfactory properties, however. In particular the crosslinking of the transparent coating (KE) is inadequate.
Moreover, the pigmented coating composition used in the process described in WO 00/63015 comprises thermoplastic polymers containing dyes or pigments dispersed in the polymer layer. This color layer is applied by extrusion, while the use of solventborne or aqueous, pigmented coating compositions and their application techniques are not described.
EP-A-819 516, furthermore, discloses a process for producing moldings, in which a sheet that has been provided with a coating is inserted into a mold, the mold is closed and contacted with a polymeric material (KM) and the polymeric material (KM) is solidified, the process having the characteristic feature that the coating material is only partly crosslinked before the polymeric material is introduced, and is only crosslinked to completion during and/or after the introduction of the polymeric material (KM). Preference in the process is given to using radiation-crosslinkable coating compositions having a glass transition temperature of below 40° C., based in particular on urethanes. However, further information on the constitution of suitable coating compositions is absent.
EP-B-403 573, moreover, describes a sheet which has been provided with a coating and is intended for use in the thermoforming process, an essential feature being that a clearcoat material which is uncrosslinked or has a low degree of crosslinking, and has a glass transition temperature of below 20° C., forms at least part of the clearcoat film.
EP-B-1 144 476, finally, discloses coating compositions which are referred to as dual-cure compositions, being curable both by thermal addition and by radiation-induced addition, and discloses their use for producing thermoformable dry-paint films. The coating compositions described therein, however, contain free isocyanate groups, which are needed for the thermal cure. The coating compositions therefore have a relatively complex physical composition. Moreover, operational control is difficult when the aim is to avoid uncontrolled curing of the thermally reactive constituents during the thermal operating steps, such as the thermoforming at elevated temperature, for example.